In the field of respirators, a supply of air is provided to a patient, usually with inspiratory and expiratory cycles to aid the patient's breathing cycle. The air is supplied at a mouthpiece or through a breathing tube into the lungs and the pressure of the airway to the patient is monitored with a pressure sensor which is pneumatically coupled to the airway. The output from the pressure sensor may be an electrical signal which may be recorded for monitoring of the patient and may be used in various control functions in the respirator.
Mechanical ventilation for respiratory distress syndrome has relied heavily on constant flow, time cycled respirators. In most neonatal and pediatric intensive care centers these respirators are pressure limited with sufficient flow rates to achieve a square pressure waveform. A major limitation in pressure limited ventilation is that volume is unknown and thus sudden changes in compliance, such as occur following accidental pneumothorax or endobronchial intubation, may escape detection until a change in the patient's condition becomes clinically evident.
It has been recognized that the pressure signal from the pressure sensor represents various phases of the patient's breathing cycle. For example, with reference to an article entitled "Obtaining and Interpreting Respiratory Flow, Pressure and Volume Waveforms" by M. Saklad et al, published in the Journal of the International Anesthesiology Clinics, Volume 12, pages 25-45, 1974 an upper airway pressure wave form is shown on page 30 and which is similar to the waveform at 8 in FIG. 2 herein. The pressure-time waveform is characterized by an inspiration phase commencing at a time t.sub.1 at a baseline 10 and increasing to a peak at 12 and a plateau 14. At about a time t.sub.2, the inspiration phase ends and expiration begins following a waveform as shown, to baseline 10 until a successive inspiration phase begins at a time t.sub.3. As indicated at page 40 of this article, the portion of the inspiration phase characterized by the sloped segment of the inspiratory pressure waveform segment 16 is related to the compliance of the lung and chest wall when the pressure at the patient airway is being monitored. Hence, in a pressure limited, time cycled ventilation the lung is exposed to a pressure head and will fill with a volume of air dependent upon lung compliance.
The desirability and benefit of continuous, dynamic monitoring of the airway pressure signal from patients requiring ventilating assistance and, in particular, infants and children, is noted in an article entitled "Continuous Dynamic Monitoring Of Pressure And Flow Patterns During Assisted Ventilation" by A. G. Galvis et al, published in the Journal of Pediatric Surgery Vol. 11, No. 3, (June) 1976 at page 307.
The latter article provides a technique for displaying pressure waves, such as 8 in FIG. 2, on an oscilloscope and teaching personnel how to recognize and identify problems from the displayed waveforms. Such monitoring program, though effective, does not readily permit recognition of breathing difficulties or gradual degeneration attributable to an alteration in compliance unless these are characterized by visually distinct increase in waveform amplitude.
In an article by W. Specht et al entitled "Monitoring of Pressure-limited Neonatal Ventilators" published in Respiratory Care of January 1978, Vol. 23, No. 1 at pages 74, 76, it is recognized that the slope of the pressure wave signal represents lung compliance and, therefore, observation of the pressure-time relationships in neonates may be clinically useful in determining ventilator management. The slope measuring from an oscilloscope display or chart-recording of the pressure wave signal, however, not only is tedious but also does not provide an accurate and reliable basis for monitoring the lung compliance of the patient so that a rapid diagnosis is often needed for acute complications such as a pneumathorax condition is not available.